Distributed Objects Remote Method Invokation: Conceptual model.

Distributed Objects

Remote Method Invokation:Conceptual model

Object 1 Object 2invoke method

respond

Object Oriented ParadigmObject Oriented Paradigm

Object 1 Object 2invoke method

respond

Client Host/Process Server Host/Process

Distributed Object Oriented ParadigmDistributed Object Oriented Paradigm

Object 1 Object 2

socketinteraction

Local – Client Host/Process

Remote -Server Host/Process

“Post Office” “Post Office”

Distributed Object Oriented: implementationDistributed Object Oriented: implementation

Object 1 Object 2

magic

Local –Client Host/Process

Remote- Server Host/Process

Stub of Object 2 Skeleton of Object 2

Distributed Object Oriented: RMI paradigmDistributed Object Oriented: RMI paradigm

Distributed Objects

A “do it yourself” implementation

Un oggetto distribuito “fai da te”

package distributedobjectdemo;

public interface Person { public int getAge() throws Throwable; public String getName() throws Throwable;}

1. Person: l’interfaccia



public class PersonServer implements Person{ int age; String name; public PersonServer(String name,int age){ this.age=age; this.name=name; } public int getAge(){ return age; } public String getName(){ return name; } public static void main(String a[]) { PersonServer person = new PersonServer("Marko", 45); Person_Skeleton skel = new Person_Skeleton(person); skel.start(); System.out.println("server started"); }}

2. Person: la classe


package distributedobjectdemo;import java.net.Socket;import java.net.ServerSocket;import java.io.*;

public class Person_Skeleton extends Thread { PersonServer myServer; int port=9000;

public Person_Skeleton(PersonServer server) { this.myServer=server; }// la classe continua…

3. Person: lo skeleton


public void run(){ Socket socket = null; ServerSocket serverSocket=null; try { serverSocket=new ServerSocket(port); } catch (IOException ex) { System.err.println("error while creating serverSocket"); ex.printStackTrace(System.err); System.exit(1); }

while (true) { try { socket=serverSocket.accept(); System.out.println("Client opened connection"); } catch (IOException ex) { System.err.println("error accepting on serverSocket"); ex.printStackTrace(System.err); System.exit(1); } // il metodo continua…



try { while (socket!=null){ ObjectInputStream instream=

new ObjectInputStream(socket.getInputStream()); String method=(String)instream.readObject(); if (method.equals("age")) { int age=myServer.getAge(); ObjectOutputStream outstream=

new ObjectOutputStream(socket.getOutputStream()); outstream.writeInt(age); outstream.flush(); } else if (method.equals("name")) { String name=myServer.getName(); ObjectOutputStream outstream=

new ObjectOutputStream(socket.getOutputStream()); outstream.writeObject(name); outstream.flush(); } } //prosegue con il catch…



} catch (IOException ex) { if (ex.getMessage().equals("Connection reset")) { System.out.println("Client closed connection"); } else { System.err.println("error on the network"); ex.printStackTrace(System.err); System.exit(2); } } catch (ClassNotFoundException ex) { System.err.println("error while reading object from the net"); ex.printStackTrace(System.err); System.exit(3); } }//fine del ciclo while(true) } //fine del metodo run} //fine della classe



package distributedobjectdemo;import java.net.Socket;import java.io.*;

public class Person_Stub implements Person { Socket socket; String machine="localhost"; int port=9000;

public Person_Stub() throws Throwable { socket=new Socket(machine,port); } protected void finalize(){ System.err.println("closing"); try { socket.close(); } catch (IOException ex) {ex.printStackTrace(System.err); } } // la classe continua…

4. Person: lo stub


public int getAge() throws Throwable { ObjectOutputStream outstream=

new ObjectOutputStream(socket.getOutputStream()); outstream.writeObject("age"); outstream.flush(); ObjectInputStream instream=

new ObjectInputStream(socket.getInputStream()); return instream.readInt(); }

public String getName() throws Throwable { ObjectOutputStream outstream=new ObjectOutputStream(socket.getOutputStream()); outstream.writeObject("name"); outstream.flush(); ObjectInputStream instream=

new ObjectInputStream(socket.getInputStream()); return (String)instream.readObject(); }} // fine della classe

4. Person: lo stub



public class Client {

public Client() { try { Person person=new Person_Stub(); int age=person.getAge(); String name=person.getName(); System.out.println(name+" is "+age+" years old"); } catch (Throwable ex) { ex.printStackTrace(System.err); } } public static void main(String[] args) { Client client1 = new Client(); }}

5. Person: il client

Open issues

-multiple instances-Automatic stub and skeleton generation-on demand server dentification -on demand remote class activation

Client Broker Server

Registry

Distributed Objects

An RMI basic implementation

CLIENT & SERVER: iCalendar (interface)

import java.rmi.*;public interface iCalendar extends Remote { java.util.Date getDate () throws RemoteException;}

1. Define the common interface

SERVER: CalendarImpl

import java.util.Date;import java.rmi.*;import java.rmi.registry.*;import java.rmi.server.*;public class CalendarImpl extends UnicastRemoteObject implements iCalendar { public CalendarImpl() throws RemoteException {} public Date getDate () throws RemoteException { return new Date(); } public static void main(String args[]) {

CalendarImpl cal; try { LocateRegistry.createRegistry(1099); cal = new CalendarImpl(); Naming.bind("rmi:///CalendarImpl", cal); System.out.println("Ready for RMI's"); } catch (Exception e) {e.printStackTrace();} }}

2. Implement the service



CalendarImpl cal; try { LocateRegistry.createRegistry(1099); cal = new CalendarImpl(); Naming.bind("rmi:///CalendarImpl", cal); System.out.println("Ready for RMI's"); } catch (Exception e) {e.printStackTrace()} }}

3. Create Registry



CalendarImpl cal; try { LocateRegistry.createRegistry(1099); cal = new CalendarImpl(); Naming.bind("rmi:///CalendarImpl", cal); System.out.println("Ready for RMI's"); } catch (Exception e) {e.printStackTrace()} }}

4. Register yourself

Server

It is not necessary to have a thread wait to keep the server alive. As long as there is a reference to the CalendarImpl object in another virtual machine, the CalendarImpl object will not be shut down or garbage collected. Because the program binds a reference to the CalendarImpl in the registry, it is reachable from a remote client, the registry itself!The CalendarImpl is available to accept calls and won't be reclaimed until its binding is removed from the registry, and no remote clients hold a remote reference to the CalendarImpl object.

CLIENT: CalendarUser

import java.util.Date; import java.rmi.*;public class CalendarUser { public static void main(String args[]) { long t1=0,t2=0; Date date; iCalendar remoteCal; try { remoteCal = (iCalendar) Naming.lookup("rmi://HOST/CalendarImpl"); t1 = remoteCal.getDate().getTime(); t2 = remoteCal.getDate().getTime(); } catch (Exception e) {e.printStackTrace();} System.out.println("This RMI call took " + (t2-t1) + " milliseconds" ); }}

6. Use Service

Preparing and executing

SERVERC:dirCalendarImpl.javaiCalendar.java

C:javac CalendarImpl.javaC:rmic CalendarImpl

C:dirCalendarImpl.javaiCalendar.javaCalendarImpl.classiCalendar.class CalendarImpl_Stub.classCalendarImpl_Skel.class

C:java CalendarImpl

CLIENT

C:dirCalendarUser.javaiCalendar.java

C:javac CalendarUser.java

C:dirCalendarUser.javaiCalendar.javaCalendarImpl_Stub.class

C:java CalendarUser

copy

Preparing and executing (version in package rmidemo)

SERVERC:dir rmidemoCalendarImpl.javaiCalendar.java

C:javac rmidemo/CalendarImpl.javaC:rmic rmidemo.CalendarImpl

C:dir rmidemoCalendarImpl.javaiCalendar.javaCalendarImpl.classiCalendar.class CalendarImpl_Stub.classCalendarImpl_Skel.class

C:java rmidemo.CalendarImpl

CLIENT

C:dir rmidemoCalendarUser.javaiCalendar.java

C:javac rmidemo/CalendarUser.java

C:dir rmidemoCalendarUser.javaiCalendar.javaCalendarImpl_Stub.class

C:java rmidemo.CalendarUser

copy

Distributed Objects

An RMI implementation - Addendum -

Preparing and executing - security

The JDK 1.2 security model is more sophisticated than the model used for JDK 1.1. JDK 1.2 contains enhancements for finer-grained security and requires code to be granted specific permissions to be allowed to perform certain operations.

Since JDK 1.2, you need to specify a policy file when you run your server and client.

grant { permission java.net.SocketPermission "*:1024-65535", "connect,accept"; permission java.io.FilePermission "c:\\…path…\\", "read"; };

java -Djava.security.policy=java.policy executableClass

Accesso alle proprietà di sistema

Nota: instead of specifìying a property at runtime (-D switch of java command), You can hardwire the property into the code:

-Djava.security.policy=java.policy

System.getProperties().put( "java.security.policy", "java.policy");

Preparing and executing

NOTE: in Java 2 the skeleton may not exist (its functionality is absorbed by the class file).

In order to use the Java 2 solution, one must specify the switch –v1.2

C:rmic –v1.2 CalendarImpl

IMPORTANT: Parameter passing

Java Standard: void f(int x) :Parameter x is passed by copyvoid g(Object k) : Parameter k and return value are passed by reference

Java RMI:void h(Object k) : Parameter k is passed by copy!UNLESS k is a REMOTE OBJECT (in which case it is passed as a REMOTE REFERENCE, i.e. its stub is copied if needed)

IMPORTANT: Parameter passing

Passing By-ValueWhen invoking a method using RMI,all parameters to the remote method are passed by-value .This means that when a client calls a server,all parameters are copied from one machine to the other.

Passing by remote-referenceIf you want to pass an object over the network by-reference,it must be a remote object, and it must implement java.rmi.Remote.A stub for the remote object is serialized and passed to the remote host. The remote host can then use that stub to invoke callbacks on your remote object. There is only one copy of the object at any time,which means that all hosts are calling the same object.

Serialization

•Any basic primitive type (int,char,and so on) is automatically serialized with the object and is available when deserialized.

•Java objects can be included with the serialized or not:

•Objects marked with the transient keyword are not serialized with the object and are not available when deserialized.

•Any object that is not marked with the transient keyword must implement java.lang.Serializable .These objects are converted to bit-blob format along with the original object. If your Java objects are neither transient nor implement java.lang.Serializable ,a NotSerializable Exception is thrown when writeObject()is called.

When not to Serialize

•The object is large.Large objects may not be suitable for serialization because operations you do with the serialized blob may be very intensive. (one could save the blob to disk or transporting the blob across the network)

•The object represents a resource that cannot be reconstructed on the target machine.Some examples of such resources are database connections and sockets.

•The object represents sensitive information that you do not want to pass in a serialized stream..

Alternatives – starting the register

Instead of writing in the server code:LocateRegistry.createRegistry(1099);

You can satrt the registry from the shell: C: rmiregistry 1099 (port number is optional)

Note: in Java 2 you need an additional parameter:C: rmiregistry –J-Djava.security.policy=registerit.policy

where registerit.policy is a file containing:

grant {permission java.security.AllPermission}

Or some permission restriction. Typically the file is kept in %USER_HOME%/.java.policy

RMI-IIOP

RMI-IIOP is a special version of RMI that is compliant with CORBA and uses both java.rmi and javax.rmi .

RMI has some interesting features not available in RMI-IIOP,such as distributed garbage collection, object activation,and downloadable class files.

But EJB and J2EE mandate that you use RMI-IIOP, not RMI.

Distributed Objects

dynamic stub loading

Alternative 2 – stub dynamic loading

Instead of manually copying the stub from theServer to client, can we automatically load the stub at runtime?

“RMI can download the bytecodes of an object's class if the class is not defined in the receiver's virtual machine. The types and the behavior of an object, previously available only in a single virtual machine, can be transmitted to another, possibly remote, virtual machine. RMI passes objects by their true type, so the behavior of those objects is not changed when they are sent to another virtual machine. This allows new types to be introduced into a remote virtual machine, thus extending the behavior of an application dynamically.”

Alternativa 2 – caricamento dinamico dello stub

SERVER

rmiregistry

CLIENT

http Server STUB

VM -rmi class

VM –client code

This client expects a URL in the marshalling stream for the remote object. It will load the stub class for the remote object from the URL in the marshalling stream. Before you can load classes from a non-local source, you need to set a security manager.Note, as an alternative to using the RMISecurityManager, you can create your ownsecurity manager.

import java.util.Date; import java.rmi.*;public class CalendarUser { public static void main(String args[]) { long t1=0,t2=0; Date date; iCalendar remoteCal; System.setSecurityManager(new RMISecurityManager()); try { remoteCal = (iCalendar) Naming.lookup("rmi://HOST/CalendarImpl"); t1 = remoteCal.getDate().getTime(); t2 = remoteCal.getDate().getTime(); } catch (Exception e) {e.printStackTrace();} System.out.println("This RMI call took " + (t2-t1) + “ milliseconds“ ); }}

CLIENT: CalendarUser-Caric.dinamic

o

SERVER: CalendarImpl – Caricamento dinamico

import java.util.Date;import java.rmi.*;import java.rmi.registry.*;import java.rmi.server.*;public class CalendarImpl extends UnicastRemoteObject implements iCalendar { public CalendarImpl() throws RemoteException {} public Date getDate () throws RemoteException { return new Date(); }

public static void main(String args[]) { CalendarImpl cal; System.setSecurityManager(new RMISecurityManager()); System.getProperties().put( "java.rmi.server.codebase", "http://HOST/java/classes/"); try { LocateRegistry.createRegistry(1099); cal = new CalendarImpl(); Naming.bind("rmi:///CalendarImpl", cal); System.out.println("Ready for RMI's"); } catch (Exception e) {e.printStackTrace()} }}

The first part remains untouched

Distributed Objects

A different paradigm:dynamic loading of a remote class

Dynamic loading of a remote class

Object 1

Class 21. Get class

Client Host Server Host

Class 2

Object 2

2. Create instance

3. Invoke method

Different paradigm:1. Laod at runtime a

class from a remote machine,

2. Create a local instance

3. Execute it.

An utility class: a quitter Window

package rmiDynamicLoadingDemo;

import java.awt.event.*;import javax.swing.*;

public class QuitterJFrame extends JFrame { //Overridden so we can exit when window is closed protected void processWindowEvent(WindowEvent e) { super.processWindowEvent(e); if (e.getID() == WindowEvent.WINDOW_CLOSING) { System.exit(0); } }}

A remote class

package rmiDynamicLoadingDemo;import javax.swing.*;import java.awt.*;public class NetworkApp implements Executable { JFrame f; public NetworkApp(QuitterJFrame f) { this.f = f; }; public void exec() { f.setBackground(Color.DARK_GRAY); f.setForeground(Color.white); JLabel l = new JLabel("Latest version of your application.", JLabel.CENTER); f.getContentPane().add("Center",l); f.pack(); f.repaint(); }}

package rmiDynamicLoadingDemo;

public interface Executable {

public void exec();

}

package rmiDynamicLoadingDemo;import javax.swing.*;import java.net.URL; import java.rmi.RMISecurityManager;import java.rmi.server.RMIClassLoader; import java.lang.reflect.*;import java.security.Permission;

public class ExecutableLoader { public static void main(String args[]) { System.setSecurityManager( new RMISecurityManager() { public void checkPermission(Permission p){} });;

Caricatore dinamico-1

JFrame cf = new QuitterJFrame(); cf.setTitle("NetworkApp"); // download a class from the net, and create an instance of it try { URL url = new URL("http://latemar.science.unitn.it/java/"); Class cl =

RMIClassLoader.loadClass(url,"rmiDynamicLoadingDemo.NetworkApp");

Class argTypes[] = {cf.getClass()}; Object argArray[] = {cf}; // create an instance of cl using constructor cntr Constructor cntr = cl.getConstructor(argTypes); Executable client = (Executable)cntr.newInstance(argArray);

client.exec(); cf.show(); } catch (Exception e) {e.printStackTrace();} }}

Caricatore dinamico-2

Distributed Objects Remote Method Invokation: Conceptual model.

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